Various experiments are proposed dealing with the mechanism by which Epstein-Barr virus transforms lymphocytes into continuous lymphoblastoid cell lines. Efforts will be made to distinguish factors which promote transformation of growth properties (immortalization) and factors which affect transformation of the cell surface. Among host cell factors which will be studied are the age and species of the cell donor, the subpopulations of lymphoid cells which are susceptible and the physiologic state of the host cell at the time of virus exposure. These problems will be studied by means by quantitative assays such as the DNA stimulation test and the "transformed centers" technique with which it is possible to determine the fraction of a cell population which is transformed.